Tuberculosis (TB) remains the leading cause of preventable deaths in the world with 100 million new infections and two million deaths each year. TB is caused by Mycobacterium tuberculosis (hereinafter also referred to by the abbreviation “Mtb”), an acid-fast bacillus that is transmitted primarily via the respiratory route. The aerosol containing the pathogen is released from people with active TB when they cough or sneeze. When a person breathes in the pathogen it enters the alveolar macrophages via a variety of receptors. Mtb multiplies within the vacuoles in the macrophage, avoids fusion with the acidic lysosomes and eludes the host defenses. As the host defense system senses the multiplying pathogen and mounts its immune defense, the pathogen goes into a non-replicating, drug-resistant, latent state. The protective response by the immune system at the site of infection results in the formation of a granuloma that contains the infection and prevents its spread. Live bacilli have reportedly been isolated from granulomas or tubercles in the lungs of persons with clinically inactive tuberculosis, regarded as the latent form of TB, indicating that the organism can persist in granulomatous lesions for decades. It is estimated that one-third of the world population has latent TB. These individuals are asymptomatic latent carriers who exhibit no signs of disease. Their risk for reactivation is estimated to be 2-23% over their life time. One study concluded that a 25 year old with latent TB has a 7.3% life time risk of reactivation. The risk increases dramatically for persons coinfected with HIV, more like 10% per year. Thus, the advent of AIDS greatly amplified the TB threat to human health. The deadly partnership between TB and AIDS, especially with multi- and extremely drug-resistant TB, is contributing to a dramatic rise in TB cases worldwide leading to a grave situation. The emergence and spread of multi-drug resistant and extremely drug-resistant TB is widely recognized as a major threat to public health.
The ability of the pathogen to go into the drug-resistant latent state is a major road block to the eradication of TB. It is known that latent Mtb persists in a non-replicating state. Antibiotics used to treat bacterial infection are usually active against growing bacteria but not against the dormant pathogen. Correlation between antibiotic activity and bacterial growth state in streptomycin-dependent Mtb was shown almost 30 years ago. The antibiotic-resistance of non-growing bacteria is due to changes in bacterial metabolism or physiological state and is described as phenotypic resistance. The phenotypic resistance has been classified into three types based on the physiological state of bacteria as stationary phenotypic resistance, persister phenotypic resistance and phenotypic resistance in dormant bacteria. Mtb displays dormancy-related phenotypic resistance which is demonstrated by the Cornell mouse model. Traditionally, the phenotypic resistance is exemplified by resistance to the antibiotic Rifampicin (Rif) and is regarded as one of the hallmarks of latent TB. The mechanism of phenotypic resistance in dormant Mtb is not clearly understood.
Development of drugs that can effectively kill dormant Mtb is of vital importance for the eradication of TB. If such drugs would prevent the pathogen from surviving in a drug-resistant state, a combination of such drugs with currently used antibiotics could drastically shorten the period of treatment for complete cure and lead to global eradication of TB. For this purpose, we need to identify processes that are necessary for the pathogen to go into dormancy, survive under the nonreplicating drug-resistant state, and get reactivated when the immune system of the host is weakened. Such steps, essential for the latent pathogen, could offer ideal targets for novel antilatency drugs that can eliminate the dormant pathogen. To achieve these objectives we explored the biochemical processes that the pathogen uses to survive for such long periods under a latent state. It has been known for many decades that Mtb in the host uses fatty acids as the major source of energy. It is well known that glyoxylate cycle is used by organisms that live on fatty acids. In recent years the important role of isocitrate lyase, a key enzyme uniquely used in the glyoxylate cycle, was shown to be required for the persistence of Mtb in the host demonstrating the central role played by fatty acid catabolism in persistence. However, the source of fatty acids used by the pathogen remains unclear. We postulated that the pathogen probably stores energy as triacylglycerol (TG) as it goes into dormancy and uses this stored energy to survive the long dormant period at very low metabolic rates as many living organisms such as hibernating animals, seeds and spores do for similar purposes. We began to identify the likely gene products that the pathogen uses to store TG and to release the fatty acids for catabolism. We also initiated the development of an in vitro dormancy model to test the hypothesis that lipid storage and mobilization are of importance for latency, a model that can be adapted for screening antilatency drug candidates.
TG is an important storage form of lipid that accumulates in species belonging to the actinomycetes family, particularly Mtb. Intracellular TG inclusion bodies were detected in mycobacteria isolated from organ lesions and Mycobacterium bovis BCG was reported to preferentially use TG within macrophages indicating that TG is probably used as an energy source by Mtb during the course of the disease. We have shown that TG accumulates when Mtb is subjected to hypoxia or nitric oxide treatment that led to a dormancy-like state in culture. We identified fifteen members of a novel class of diacylglycerol acyltransferase genes which we designated as tgs (triacylglycerol synthase). Several of the tgs genes were significantly upregulated under hypoxic conditions and under nitric oxide treatment, particularly those that show the highest TG synthase activity when expressed in E coli. We identified Rv3130c as the prime gene in the biosynthesis of TG in the bacterium under in vitro dormancy-like conditions. Our hypothesis was strongly supported by a important recent report on the W/Beijing lineage of Mtb strains which has been associated with the increasing incidence of multi-drug resistant (MDR) TB epidemic in Asia. The W/Beijing strains were shown to overproduce TG and the Rv3130c gene was constitutively upregulated along with the dormancy regulator protein DosR. The authors suggested that constitutive accumulation of TG by this strain may confer an adaptive advantage for growth in microaerophilic or anaerobic environments and thus be related to the epidemiological spread of this strain. Our hypothesis concerning the importance of Rv3130c is strongly supported by the remarkable finding by our collaborators. A recently developed two step multiplex and real time PCR method was adapted for reliable quantitative gene profiling of the small amount of latent Mtb expected to be found in infected animal and human host lung tissues. Remarkably, tgs1 (Rv3130c) was by far the most upregulated gene in the pathogen within the host, while dosR and aceAa that are well-known to be involved in dormancy, were much less induced. Many organisms use waxy esters (WE) as the major form of energy storage. Mtb also stores WE but the genes involved in the synthesis of WE and the growth conditions that cause its accumulation have not been identified. The basic mechanisms used for biosynthesis of WE were first elucidated in our laboratory several decades ago and the enzymatic strategy described more recently. We have recently shown that Rv3391 and Rv1543 encode acyl-CoA reductases involved in WE synthesis in Mtb. Rv3391 has been reported to be upregulated under nutrient stress conditions. We found that WE accumulates under stress conditions that lead to a dormancy-like state and the accumulated WE is utilized upon starvation. This utilization was reduced in lipY mutant, indicating the involvement of lipY in WE hydrolysis. Thus, Mtb can produce and use both major energy storage forms, TG and WE, and both forms are likely to be used for successfully going through dormancy. WE may also be a component of the cell wall lipids that control permeability.